A mounting structure manufactured by the flip chip method includes a semiconductor chip, e.g., an LSI chip mounted on a circuit board. In the flip chip method, protruding electrodes (bumps) such as solder bumps are formed on the electrode terminals (electrode pads) of a semiconductor chip, and then the semiconductor chip is mounted face down on a circuit board. Specifically, in a flip-chip process, the electrode terminals of the semiconductor chip are aligned with the electrode terminals of the circuit board, and then the protruding electrodes on the heated semiconductor chip are pressed to the electrode terminals of the circuit board.
Generally, solder bumps are formed as follows: solder is fed onto the electrode terminals of a semiconductor chip by screen printing, a dispenser, or electrolytic plating, and then the semiconductor chip is heated at least to the melting point of solder in a reflow furnace. If the protruding electrodes are solder bumps, a clearance between the semiconductor chip and the circuit board is filled with molding resin to enhance bonding strength between the electrode terminals of the semiconductor chip and the electrode terminals of the circuit board.
In addition to solder bumps, protruding electrodes made of gold or copper are available. Protruding electrodes made of gold or copper are generally formed by electrolytic plating. In the case of protruding electrodes made of gold or copper, generally, an anisotropic conductive film with an adhesive containing metallic particles is temporarily bonded onto a circuit board, and then a semiconductor chip is bonded face down onto the circuit board via the anisotropic conductive film by thermocompression bonding.
In order to increase the number of pins with higher densities on semiconductor chips, the pitches of electrode terminals on semiconductor chips have been reduced with a smaller area. The pitches of electrode terminals, in particular, have been considerably reduced. Thus, in the case of conventional electrode terminals arranged in a row or staggered in two rows on the outer periphery of a semiconductor chip, a short circuit may occur between the adjacent electrode terminals or faulty connection may occur due to a thermal stress resulting from a difference in thermal expansion coefficient between a semiconductor chip and a circuit board.
Specifically, if the protruding electrodes are solder bumps, a so-called faulty bridge may occur, leading to a short circuit between the adjacent electrode terminals. Such a faulty bridge occurs when molten solder is deformed to connect the adjacent solder bumps with a surface tension of solder in a flip-chip process. Hence, as the pitches of electrode terminals of semiconductor chips decrease, faulty connections are more likely to occur. Moreover, a faulty connection, which is caused by a thermal stress resulting from a difference in thermal expansion coefficient between a semiconductor chip and a circuit board, appears because the narrow pitches of the electrode terminals of the semiconductor chip prevent molding resin in a clearance between the semiconductor chip and the circuit board from filling all gaps between solder bumps placed on the outer periphery of the semiconductor chip.
In the case of protruding electrodes made of gold or copper, as the pitches of electrode terminals of a semiconductor chip decrease, metallic particles dispersed in an anisotropic conductive film may join to one another between the adjacent protruding electrodes and cause a short circuit between the adjacent electrode terminals. Moreover, a faulty connection, which is caused by a thermal stress resulting from a difference in thermal expansion coefficient between the semiconductor chip and a circuit board, appears because the narrow pitches of the electrode terminals of the semiconductor chip prevent the anisotropic conductive film from filling all gaps between the protruding electrodes placed on the outer periphery of the semiconductor chip.
As has been discussed, as the pitches of electrode terminals of a semiconductor chip decrease, a short circuit may occur between the adjacent electrode terminals or faulty connection may occur due to a thermal stress resulting from a difference in thermal expansion coefficient between the semiconductor chip and a circuit board. Hence, in order to increase the pitches between the electrode terminals, the electrode terminals are disposed over the area of the major surface (element surface) of the semiconductor chip (for example, being spaced in a lattice pattern).
However, in recent years, even electrode terminals disposed over the area of the major surface of a semiconductor chip have considerably decreased in pitch, leading to a short circuit between the electrode terminals. In order to solve this problem, protruding electrodes with cylindrical surfaces are proposed (for example, see Patent literature 1). The cylindrical surface made of gold or copper is covered with an insulating coating containing metallic particles. When the protruding electrodes covered with the insulating coatings are pressed into contact with the electrode terminals of a circuit board, the insulating coatings are compressed such that metallic particles penetrate the insulating coatings into contact with the electrode terminals of the circuit board. This ensures electrical connection between the cylindrical electrodes and the electrode terminals of the circuit board. Moreover, the metallic particles do not join to one another between the adjacent protruding electrodes, preventing a short circuit between the adjacent electrode terminals.
In a mounting structure described in Patent Literature 1, however, metallic particles only come into contact with the cylindrical electrodes on a semiconductor chip and the electrode terminals of the circuit board without diffused junction. Thus, if the electrode terminals of the semiconductor chip decrease in area, the number of metallic particles decreases between the cylindrical electrodes and the electrode terminals of the circuit board. This increases connection resistance between the electrode terminals of the semiconductor chip and the electrode terminals of the circuit board. In recent years, narrower pitches and smaller areas have been seriously demanded of the electrode terminals of semiconductor chips. Thus, in the mounting structure described in Patent Literature 1, if the electrode terminals of the semiconductor chip have areas as has been recently demanded, connection resistance disadvantageously increases between the electrode terminals of the semiconductor chip and the electrode terminals of the circuit board. The increased connection resistance may increase the transmission loss of a signal transmitted between the electrode terminals of the semiconductor chip and the electrode terminals of the circuit board.
Additionally, protruding electrodes having upper electrodes of solder are proposed (for example, see Patent Literature 2). The upper electrodes are disposed on refractory lower electrodes that do not melt in a flip-chip process. The protruding electrode having a two-layer structure has a smaller amount of solder than a solder bump that is only composed of solder. This reduces the amount of solder spilling in a plane in the flip chip process, thereby reducing the occurrence of faulty bridges. Moreover, the solder of the upper electrodes makes diffused junctions with the lower electrodes and the electrode terminals of a circuit board, reducing connection resistance between the electrode terminals of a semiconductor chip and the electrode terminals of the circuit board without increasing the transmission loss of a signal transmitted between the electrode terminals of the semiconductor chip and the electrode terminals of the circuit board.